Generally, a Liquefied Petroleum Gas (LPG) fuel is injected directly into the engine in a Liquefied Petroleum Injection (LPI) fuel system for improving the engine output and reducing exhaust gas of an LPG vehicle. In the LPI fuel system, an Electronic Control Unit (ECU) of an interface box diagnoses a fuel pump base on a Pulse Width Modulation (PWM) signal inputted from a fuel pump driver. If no PWM signal is inputted into the ECU, then a signal line is determined to be disconnected and a warning lamp is illuminated.
However, one drawback in a conventional LPI fuel system is that if the user turns off an LPG switch while driving, thus preventing the PWM signal from being inputted into the interface box, the ECU of the interface box still determines that the signal line is disconnected and erroneously illuminates the warning lamp.
Another drawback in a conventional LPI fuel system is that a diode is connected to a cut-off solenoid valve or the like to protect the system from a voltage surge generated according to the on/off operation of the LPG switch. This complicates the configuration of the diagnosis system and increases manufacturing costs.